you don't have to register the business name, but you do have to register your business within 3 months with the inland revenue (oops haven't done mine yet, this has reminded me!!!) Phone 0845 9154515 (new self employed helpline) and they'll send you a pack out. This even applies if your not earning much, they also send you a form out so you don't have to pay national insurance if your earnings are below a certain amount.